1. Field of the Invention
The present invention relates in general to a printed circuit board on which electronic components such as chips are mounted.
2. Description of the Prior Art
When an electronic component is mounted on a printed circuit board, soldering process such as reflowing is often employed in the surface mount technology. In the process of soldering, the printed circuit board is subjected to a high temperature atmosphere of 200-230 Celsius degrees. When the printed circuit board is cooled after the process of soldering, the printed circuit board is subjected to shrinkage scaled by a thermal expansion coefficient. The printed circuit board suffers from a smaller shrinkage at the front surface, on which electronic components are mounted, and a larger shrinkage at the back surface. Such uneven or irregular shrinkage induces a warp of the printed circuit board. The warp sometimes causes the mounted electronic components to be torn apart from the printed circuit board. The warp also sometimes serves to generate a larger stress in the input/output pins of the electronic components, which may deteriorate the durability of the input/output pins, namely, the mounted electronic components. Heretofore, a high rigidity linear or planar member of metallic material is often coupled to the back surface of the printed circuit board so as to cancel the warp. When the printed circuit board is urged against the linear or planar member, the front surface of the printed circuit board can be flattened.
Large-sized computers such as supercomputers and main frames in general employ a large-sized print circuit board on which electronic components such as MCMs (multichip modules) and memory modules are mounted. The large-sized printed circuit board tends to deform because of its size, so that it is difficult to keep the front surface of the large-sized printed circuit board plane or flat. The input/output pins of electronic components hardly contact the input/output pads on the front surface of the large-sized printed circuit board when the electronic components are to be mounted on the large-sized printed circuit board.
In particular, a printed circuit board comprising multilayered copper printed circuit patterns usually suffers from reduction in thickness at the periphery of the printed circuit board. When the copper printed circuit patterns or films are laminated or superposed in fabricating the printed circuit board, a fluid resin material often leaks out at the periphery of the copper films, although the resin material is designed to stay between the copper films. Such leakage causes reduction in thickness of the printed circuit board. Moreover, the reduction is gradually intensified toward the periphery. Accordingly, a curved surface is formed at the front and back surfaces of the printed circuit board so as to swell at the central area of the surfaces. If the aforementioned linear or planar member is coupled to the printed circuit board at the curved back surface, the curvature of the front surface may be intensified, so that some of the input/output pins of the electronic components cannot at all touch the input/output pads on the front surface of the printed circuit board.
It is accordingly an object of the present invention to provide a printed circuit board reliably achieving electric connection to electronic components mounted thereon.
According to a first aspect of the present invention, there is provided a printed circuit board unit comprising: a printed circuit board; and a correction member contacting a surface of the printed circuit board at least three of support points arranged along a curved surface.
In addition, according to a second aspect of the present invention, there is provided a printed circuit board unit comprising: a printed circuit board; and a correction member defining a support point arranged between a pair of contact points, contacting a surface of the printed circuit board, at a position withdrawing from a plane including the contact points so as to contact the surface of the printed circuit board.
The correction members of the inventions are adapted to generate an intentional warp or bend of the printed circuit board. Such warp serves to correct or modify the curvature of the front surface of the printed circuit board. The front surface of the printed circuit board can be maintained flat. Electric connection can reliably be established between the printed circuit board and electronic components such as MCMs (multichip modules) to be mounted on the printed circuit board.
The printed circuit board unit may further comprise a coupler provided between the printed circuit board and the correction member so as to cause deformation of the printed circuit board. The coupler serves to keep urging the printed circuit board against the correction member, to that the aforementioned warp or deformation of the printed circuit board can be maintained. Accordingly, the correction member is required to have a rigidity enough to maintain its shape when the member is coupled to the printed circuit board by the coupler.
Here, the distance between the support point and the plane is preferably set to reflect the amount of shift generated at the support point when the printed circuit board and the correction member are supported at periphery in a horizontal attitude. In general, the printed circuit board is often supported at the periphery or opposite ends in a horizontal attitude when the electronic components are being mounted on the printed circuit board. A warp is generated in the printed circuit board due to the gravity of the electronic components mounted on the printed circuit board as well as the gravity of the printed circuit board itself. The aforementioned distance may serve to cancel such warp caused by the gravity. A flat surface can be maintained on the printed circuit board so as to completely receive the electronic components.
It is preferable that the correction member contacts a back surface of the printed circuit board at the contact and support points so as to correct the curvature of the front surface of the printed circuit board. The front surface can be flattened to receive electronic components. The correction member hardly hinders removal or exchange of the mounted electronic components. The maintenance of the electronic components can easily be achieved.
In this case, it is preferable that the distance measured from the support point and the plane is set to reflect a difference between a thickness of the printed circuit board at the support point and a thickness of the printed circuit board at the contact point. Such distance serves to correct or cancel the curvature of the front surface even when the curvature is caused by the variation in thickness of the printed circuit board.
The correction member can be formed into any shape or any structure as long as enough rigidity is established. For example, the correction member may be formed into a frame. In any event, the support point may be defined along a curved surface formed on the correction member or at an edge of a step formed on the correction member.
Furthermore, the correction member may comprise a main body with a flat surface opposed to the printed circuit board, and a filler interposed between the main body and the printed circuit board so as to induce a bend of the printed circuit board. Such arrangement allows employment of a conventional linear or planar member as the main body. Accordingly, the surface of the printed circuit board can be flattened without inducing a serious change in the design.
The material of the correction member is preferably selected to reflect a thermal expansion coefficient of the printed circuit board to be coupled to the member. For example, if a smaller difference can be found between the thermal expansion coefficient of the printed circuit board and the thermal expansion coefficient of the correction member, a warp can be restrained to the utmost in the printed circuit board even when the printed circuit board with the correction member is subjected to a high temperature atmosphere. When the printed circuit board is formed by impregnating glass fibers with fluid epoxy resin or polyimide, a stainless steel is preferably selected for the material of the correction member.
The aforementioned correction member may be integrated with a power supply member serving to supply an electric voltage to electronic components mounted on the printed circuit board, or with a connector for coupling an electronic component with the printed circuit board. The integration of this kind may serve to omit a space peculiar to an independent correction member.